E-mail Question

Before my box was doing fine with no problem but since I do the upgraded I've noticed that all of the domains under my box couldn't send or receive mail from webmail. I've tried to send a message, I received:

There was an error sending your message: sendmail [/usr/sbin/sendmail] not executable

I then looked into WHM and seeing that there were lot of email waiting to delivery but somehow it still in the queries, why? I tried to use the third party to check and send mail it work with no problem.